Method of measuring concentration of dissolved organic nitrogen in sewage

ABSTRACT

A method of measuring concentration of dissolved organic nitrogen in sewage. The method includes: filtering a sewage sample using a filter membrane; measuring the concentrations of total dissolved nitrogen (TDN), ammonia nitrogen (NH 4   + ), and nitric nitrogen (NO 3   − ) in the sewage sample, respectively designated as C TDN(I) , C NH4   +   (I)  and C NO3   −   (I) ; calculating the ratios of (C NH4   +   (I) +C NO3   −   (I) )/C TDN(I)  and C NO3   −   (I) /C NH4   +   (I) , and according to the ratios, calculating the concentration of dissolved organic nitrogen (DON) in the sewage sample.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.16/274,213, filed Feb. 12, 2019, now pending, which is acontinuation-in-part of International Patent Application No.PCT/CN2017/095851 with an international filing date of Aug. 3, 2017,designating the United States, and further claims foreign prioritybenefits to Chinese Patent Application No. 201710048270.3 filed Jan. 20,2017. The contents of all of the aforementioned applications, includingany intervening amendments thereto, are incorporated herein byreference. Inquiries from the public to applicants or assigneesconcerning this document or the related applications should be directedto: Matthias Scholl P.C., Attn.: Dr. Matthias Scholl Esq., 245 FirstStreet, 18th Floor, Cambridge, Mass. 02142.

BACKGROUND

This disclosure relates to a method of measuring concentration ofdissolved organic nitrogen in sewage.

Total dissolved nitrogen (TDN) in sewage includes dissolved inorganicnitrogen (DIN) and dissolved organic nitrogen (DON).

Conventionally, to measure the concentration of DON, the concentrationsof TDN and DIN (including ammonia nitrogen, nitrate nitrogen and nitritenitrogen) are measured, respectively, and then the difference betweenthem is calculated. The entire measurement processes are complex andinefficient, and the results are generally inaccurate.

SUMMARY

Disclosed is a method of measuring concentration of dissolved organicnitrogen in sewage.

The disclosure provides a method of measuring concentration of dissolvedorganic nitrogen in sewage, the method comprising:

-   -   (1) filtering a sewage sample using a filter membrane;    -   (2) measuring concentrations of total dissolved nitrogen (TDN),        ammonia nitrogen (NH₄ ⁺), and nitric nitrogen (NO₃ ⁻) in the        sewage sample, respectively designated as C_(TDN(I)), C_(NH4) ⁺        _((I)) and C_(NO3) ⁻ _((I));    -   (3) calculating ratios of (C_(NH4) ⁺ _((I))+C_(NO3) ⁻        _((I)))/C_(TDN(I)) and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I)), and        according to the ratios, performing one of the following to        measure concentrations of nitrogen-containing groups in the        sewage sample;        -   (3.1) when (C_(NH4) ⁺ _((I))+C_(NO3) ⁻            _((I)))/C_(TDN(I))<0.7, measuring a concentration of nitrite            nitrogen (NO₂ ⁻) in the sewage sample, designated as C_(NO2)            ⁻ _((I)), and calculating a concentration of dissolved            organic nitrogen (DON) in the sewage sample as follows:            DON=C_(TDN(I))—C_(NH4) ⁺ _((I))—C_(NO3) ⁻ _((I))—C_(NO2) ⁻            _((I));        -   (3.2) when (C_(NH4) ⁺ _((I))+C_(NO3) ⁻ _((I))/C_(TDN(I))≥0.7            and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))≥1, dialyzing the            sewage sample in a suspended dialysis bag for between 22 and            26 hours, and measuring concentrations of total dissolved            nitrogen (TDN), ammonia nitrogen (NH₄ ⁺), nitric nitrogen            (NO₃ ⁻) and nitrite nitrogen (NO₂ ⁻) in the sewage sample,            respectively designated as C_(TDN(II)), C_(NH4) ⁺ _((II)),            C_(NO3) ⁻ _((II)), and C_(NO2) ⁻ _((II)), and calculating a            concentration of dissolved organic nitrogen (DON) in the            sewage sample as follows: DON=C_(TDN(II))—C_(NH4) ⁺            _((II))—C_(NO3) ⁻ _((II))—C_(NO2) ⁻ _((II)); and        -   (3.3) when (C_(NH4) ⁺ _((I))+C_(NO3) ⁻            _((I)))/C_(TDN(I))≥0.7 and C_(NO3) ⁻ _((I))/C_(NH4) ⁺            _((I))<1, dialyzing the sewage sample in a suspended            dialysis bag for between 34 and 38 hours, and measuring            concentrations of total dissolved nitrogen (TDN), ammonia            nitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) and nitrite            nitrogen (NO₂ ⁻) in the sewage sample, respectively            designated as C_(TDN(III)), C_(NH4) ⁺ _((III)), C_(NO3) ⁻            _((III)), and C_(NO2) ⁻ _((III)), and calculating a            concentration of dissolved organic nitrogen (DON) in the            sewage sample as follows: DON=C_(TDN(III))—C_(NH4) ⁺            _((III))—C_(NO3) ⁻ _((III))—C_(NO2) ⁻ _((III)).

The filter membrane can have a pore size of 0.45 μm.

The suspended dialysis bag in (3.2) and (3.3) can be a cellulose estermembrane, hydrophilic, and can have a molecular weight cut-off (MWCO) of100-500 Da; and the hydraulic retention time of the dialysate in thesewage sample can be 4 h.

The concentrations of the total dissolved nitrogen (TDN), ammonianitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) and nitrite nitrogen (NO₂ ⁻)can be measured using potassium persulfate oxidation-ion chromatography,salicylic acid-hypochlorite spectrophotometry, ion chromatography andN-(1-naphthyl)-ethylenediamine spectrophotometry, respectively.

Advantages of the method of measuring concentration of dissolved organicnitrogen in sewage in the disclosure are summarized as below.

-   -   1. The method is efficient, and energy-saving;    -   2. The measurement method is accurate; and    -   3. The method is easy to operate, and is cost-effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a method of measuring concentration ofdissolved organic nitrogen in sewage as described in the disclosure; and

FIG. 2 is a comparison diagram of concentrations of dissolved organicnitrogen measured by conventional methods and the method as described inthe disclosure;

FIG. 3 is a diagram of measured DON concentrations of urea solution,glutamic acid solution, and bovine serum albumin (BSA) solution having asolute concentration of 1 mg/L and varying DIN/TDN ratios;

FIG. 4 is a diagram of 95% confidence intervals of the measured DONconcentrations of FIG. 3;

FIG. 5 is a diagram of removal rates of NO₃ ⁻ by dialyzing NW/NO₃ ⁻solutions using dialysates having different hydraulic retention times;and

FIG. 6 is a diagram of the high average absolute deviation of theconcentration of DON with respect to the actual concentration of DON inurea solution, glutamic acid solution, and BSA solution measured afterdialysis of different dialysis time.

DETAILED DESCRIPTION

Measurement conditions affecting the method of measuring the DONconcentration in sewage are described below in details with reference tothe drawings.

1. Influence of DIN/TDN Ratio on the Measurement of DON Concentration:

Three nitrogen-containing organic compounds having different molecularweights, including urea, glutamic acid, and bovine serum albumin (BSA),were dissolved in water at a concentration of 1 mg/L, respectively.Concentrations of the nitrogen-containing organic compounds in thesample solutions were measured without dialysis. As shown in FIG. 3,when DIN/TDN≤0.7, the measured DON concentrations of the threenitrogen-containing organic compounds are close to their actualconcentration of 1 mg/L, and the 95% confidence intervals (95% C.I.) ofthe measured DON concentrations are less than 0.4. When DIN/TDN≥0.7, themeasured DON concentrations of the nitrogen-containing organic compoundsobviously deviate from the actual concentration of 1 mg/L. For example,when DIN/TDN=0.92, the measured concentration of glutamic acid is1.82±0.88 mg/L, while 95% C.I.=1.72. FIG. 4 shows that 95% C.I.increases with the increase of DIN/TDN ratio.

The measurement of DON concentration is calculated by subtracting DIN(sum of the concentrations of NH₄ ⁺, NO₃ ⁻, and NO₂ ⁻) from TDN. Becauseeach species of inorganic nitrogen would produce a concentration errorduring the measurement, the subtraction of DIN that includesconcentrations of multiple species from TDN would amplify error in theDON measurement. Therefore, when DIN/TDN≥0.7 in the sewage, the accuracyof DON measurement is low (95% C.I.>0.4) and, therefore, dialysispretreatment is required to improve measurement accuracy.

2. Hydraulic Retention Time of Dialysates:

The inorganic nitrogen species in sewage are mainly NH₄ ⁺ and NO₃ ⁻, andadditionally NO₂ ⁻ at a low concentration. To determine the influence ofthe dialysates' hydraulic retention time on DON measurement, solutionsrespectively containing 5 mg/L of NH₄ ⁺ and 15 mg/L of NO₃ ⁻ wereprepared as sample solutions. Dialysis of the sample solutions wasconducted by using dialysates having a hydraulic retention time of 1, 2,4, 8, 12, and 24 h, respectively, for 12 or 24 h. The concentrations ofremaining NH⁴⁺ and NO₃ ⁻ in the dialysis bags were measured afterdialysis. The results are shown in FIG. 5. When the hydraulic retentiontime of the dialysate increases from 1 h to 4 h, the concentrations ofremaining NH₄ ⁺ and NO₃ ⁻ in the dialysis bag do not increasesignificantly. When the hydraulic retention time of dialysate is greaterthan 4 h, as the dialysate hydraulic retention time increases, theconcentrations of remaining NH₄ ⁺ and NO₃ ⁻ in dialysis bags changeobviously. For example, when the original concentration of NO₃ ⁻ is 15mg/L and the hydraulic retention time of the dialysate increases from 4h to 8 h, the ratio of the concentration of the remaining NO₃ ⁻ to theoriginal concentration of NO³⁻ (C/C₀) increases from 0.48 to 0.63 afterdialysis for 12 h, and increases from 0.29 to 0.34 after dialysis for 24h. Thus, shorter hydraulic retention time of the dialysate results ingreater removal rate of NH₄ ⁺ and NO₃ ⁻. When the hydraulic retentiontime of the dialysate is less than 4 h, the removal rate of NH₄ ⁺ andNO₃ ⁻ does not increase with the decrease of the dialysate's hydraulicretention time.

3. Dialysis Time:

Urea, glutamic acid, BSA, and a lyophilized sewage sample comprisingorganic nitrogen were dissolved in solutions having a variety of NO₃⁻/NH₄ ⁺ ratios. The concentrations of urea, glutamic acid, BSA, andlyophilized sewage sample in the sample solutions were 1 mg/L. Thehydraulic retention time of the dialysate was 4 h. The DONconcentrations of the prepared solutions were measured after dialysisfor different time. The results are shown in FIG. 6.

The NO₃ ⁻/NH₄ ⁺ ratio of the sample significantly affects the measuredDON concentration. As shown in FIG. 6, when the dialysis time (t) is 24h and the NO₃ ⁻/NH₄ ⁺ ratio≥1, the average absolute deviation of themeasured concentration of DON with respect to the actual concentration(1 mg/L) of the solutions of urea, glutamic acid, BSA, and thelyophilized sewage sample are smaller than those when NO₃ ⁻/NH⁴⁺<1. Inparticular, when NO₃ ⁻/NH₄ ⁺<1 and DIN/TDN=0.92, the average absolutedeviation of the measured concentration of DON with respect to theactual concentration of BSA is as high as 19.0%. The isoelectric pointof the dialysis membrane used in the dialysis is pH=2.2. When the pH isgreater than 2.2, the membrane is negatively charged and attracts thepositively charged NH₄ ⁺ so as to prevent NH₄ ⁺ from passing through thedialysis membrane, which leads to a removal rate of NH₄ ⁺ lower thanthat of NO₃ ⁻ and a high average absolute deviation of the measuredconcentration of DON with respect to the actual concentration of DONwhen NO₃ ⁻/NH⁴⁺<1.

As shown in FIG. 6, when NO₃ ⁻/NH₄ ⁺≥1, increase of the dialysis timefrom 24 h to 36 h or 60 h does not lead to significant change of theaverage absolute deviation of the measured concentration of DON withrespect to the actual concentration of DON.

To further illustrate, examples detailing a method of measuringconcentration of dissolved organic nitrogen in sewage are describedbelow. It should be noted that the following examples are intended todescribe and not to limit the description.

Example 1

1. 100 mL of municipal sewage (labeled as sample 1) was filtered using afilter membrane having a pore size of 0.45 μm.

2. The concentrations of the total dissolved nitrogen (TDN), NH₄ ⁺ andNO₃ ⁻ of the sewage were measured using potassium persulfateoxidation-ion chromatography, salicylic acid-hypochloritespectrophotometry, and ion chromatography, respectively, and wererecorded as C_(TDN(I)), C_(NH4) ⁺ _((I)) and C_(NO3) ⁻ _((I));

3. The results showed that (C_(NH4) ⁺ _((I))+C_(NO3) ⁻_((I)))/C_(TDN(I))=0.92>0.7 and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))>1. Thesewage was dialyzed in a suspended dialysis bag for 24 hours. Thesuspended dialysis bag was a cellulose ester membrane, hydrophilic, andhad a molecular weight cut-off (MWCO) of 100 Da; and the hydraulicretention time of the dialysate in the sewage sample was 4 h.Thereafter, the concentrations of the total dissolved nitrogen (TDN),ammonia nitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) and nitrite nitrogen(NO₂ ⁻) were measured using potassium persulfate oxidation-ionchromatography, salicylic acid-hypochlorite spectrophotometry, ionchromatography and N-(1-naphthyl)-ethylenediamine spectrophotometry,respectively, and recorded as C_(TDN(II)), C_(NH4) ⁺ _((II)), C_(NO3) ⁻_((II)), and C_(NO2) ⁻ _((II)), respectively.

4. The concentration of the dissolved organic nitrogen (DON) in thesewage sample was calculated as follows: DON=C_(TDN(II))—C_(NH4) ⁺_((II))—C_(NO3) ⁻ _((II))—C_(NO2)-_((II)). The DON in the sample 1 was0.80 mg/L. To ensure the accuracy and reliability of the measurement,the measurement was performed three times and the measured values wereaveraged. The results are shown in FIG. 2.

Example 2

1. 100 mL of municipal sewage extracted from the anaerobic stage of awastewater treatment plant (labeled as sample 2) was filtered using afilter membrane having a pore size of 0.45 μm.

2. The concentrations of the total dissolved nitrogen (TDN), NH₄ ⁺ andNO₃ ⁻ of the sewage were measured using potassium persulfateoxidation-ion chromatography, salicylic acid-hypochloritespectrophotometry, and ion chromatography, respectively, and wererecorded as C_(TDN(I)), C_(NH4) ⁺ _((I)) and C_(NO3) ⁻ _((I));

3. The results showed that (C_(NH4) ⁺ _((I))+C_(NO3) ⁻_((I))/C_(TDN(I))=0.73>0.7 and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))<1. Thesewage was dialyzed in a suspended dialysis bag for 34 hours. Thesuspended dialysis bag was a cellulose ester membrane, hydrophilic, andhad a molecular weight cut-off (MWCO) of 100 Da; and the hydraulicretention time of the dialysate in the sewage sample was 4 h.Thereafter, the concentrations of the total dissolved nitrogen (TDN),ammonia nitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) and nitrite nitrogen(NO₂ ⁻) were measured using potassium persulfate oxidation-ionchromatography, salicylic acid-hypochlorite spectrophotometry, ionchromatography and N-(1-naphthyl)-ethylenediamine spectrophotometry,respectively, and recorded as C_(TDN(III)), C_(NH4) ⁺ _((III)), C_(NO3)⁻ _((III)), and C_(NO2) ⁻ _((III)), respectively.

4. The concentration of the dissolved organic nitrogen (DON) in thesewage sample was calculated as follows: DON=C_(TDN(III))—C_(NH4) ⁺_((III))—C_(NO3) ⁻ _((III)). The DON in the sample 2 was 2.43 mg/L. Toensure the accuracy and reliability of the measurement, the measurementwas performed three times and the measured values were averaged. Theresults are shown in FIG. 2.

Example 3

1. 100 mL of municipal sewage extracted from the aerobic stage of awastewater treatment plant (labeled as sample 3) was filtered using afilter membrane having a pore size of 0.45 μm.

2. The concentrations of the total dissolved nitrogen (TDN), NH₄ ⁺ andNO₃ ⁻ of the sewage were measured using potassium persulfateoxidation-ion chromatography, salicylic acid-hypochloritespectrophotometry, and ion chromatography, respectively, and wererecorded as C_(TDN(I)), C_(NH4) ⁺ _((I)) and C_(NO3) ⁻ _((I));

3. The results showed that (C_(NH4) ⁺ _((I))+C_(NO3) ⁻_((I)))/C_(TDN(I))=0.75>0.7 and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))≥1. Thesewage was dialyzed in a suspended dialysis bag for 24 hours. Thesuspended dialysis bag was a cellulose ester membrane, hydrophilic, andhad a molecular weight cut-off (MWCO) of 100 Da; and the hydraulicretention time of the dialysate in the sewage sample was 4 h.Thereafter, the concentrations of the total dissolved nitrogen (TDN),ammonia nitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) and nitrite nitrogen(NO₂ ⁻) were measured using potassium persulfate oxidation-ionchromatography, salicylic acid-hypochlorite spectrophotometry, ionchromatography and N-(1-naphthyl)-ethylenediamine spectrophotometry,respectively, and recorded as C_(TDN(II)), C_(NH4) ⁺ _((II)), C_(NO3) ⁻_((II)), and C_(NO2) ⁻ _((II)), respectively.

4. The concentration of the dissolved organic nitrogen (DON) in thesewage sample was calculated as follows: DON=C_(TDN(II))—C_(NH4) ⁺_((II))—C_(NO3) ⁻ _((II))—C_(NO2)-_((II)). The DON in the sample 3 was1.89 mg/L. To ensure the accuracy and reliability of the measurement,the measurement was performed three times and the measured values wereaveraged. The results are shown in FIG. 2.

Example 4

1. 100 mL of treated water extracted from the output of a wastewatertreatment plant (labeled as sample 4) was filtered using a filtermembrane having a pore size of 0.45 μm.

2. The concentrations of the total dissolved nitrogen (TDN), NH₄ ⁺ andNO₃ ⁻ of the sewage were measured using potassium persulfateoxidation-ion chromatography, salicylic acid-hypochloritespectrophotometry, and ion chromatography, respectively, and wererecorded as C_(TDN(I)), C_(NH4) ⁺ _((I)) and C_(NO3) ⁻ _((I));

3. The results showed that (C_(NH4) ⁺ _((I))+C_(NO3) ⁻_((I)))/C_(TDN(I))=0.95>0.7 and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))≥1. Thesewage was dialyzed in a suspended dialysis bag for 25 hours. Thesuspended dialysis bag was a cellulose ester membrane, hydrophilic, andhad a molecular weight cut-off (MWCO) of 100 Da; and the hydraulicretention time of the dialysate in the sewage sample was 4 h.Thereafter, the concentrations of the total dissolved nitrogen (TDN),ammonia nitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) and nitrite nitrogen(NO₂ ⁻) were measured using potassium persulfate oxidation-ionchromatography, salicylic acid-hypochlorite spectrophotometry, ionchromatography and N-(1-naphthyl)-ethylenediamine spectrophotometry,respectively, and recorded as C_(TDN(II)), C_(NH4) ⁺ _((II)), C_(NO3) ⁻_((II)), and C_(NO2) ⁻ _((II)), respectively.

4. The concentration of the dissolved organic nitrogen (DON) in thesewage sample was calculated as follows: DON=C_(TDN(II))—C_(NH4) ⁺_((II))—C_(NO3) ⁻ _((II))—C_(NO2)-_((II)). The DON in the sample 4 was0.51 mg/L. To ensure the accuracy and reliability of the measurement,the measurement was performed three times and the measured values wereaveraged. The results are shown in FIG. 2.

Example 5

1. 100 mL of 0.99 mg/L standard glutamate solution was mixed with 10 mLof 40.09 mg/L potassium nitrate solution. The mixture (labeled as sample5) was filtered using a filter membrane having a pore size of 0.45 μm.

2. The concentrations of the total dissolved nitrogen (TDN), NH₄ ⁺ andNO₃ ⁻ of the mixture were measured using potassium persulfateoxidation-ion chromatography, salicylic acid-hypochloritespectrophotometry, and ion chromatography, respectively, and wererecorded as C_(TDN(I)), C_(NH4) ⁺ _((I)) and C_(NO3) ⁻ _((I));

3. The results showed that (C_(NH4) ⁺ _((I))+C_(NO3) ⁻_((I)))/C_(TDN(I))=0.81>0.7 and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))>1. Thesewage was dialyzed in a suspended dialysis bag for 24 hours. Thesuspended dialysis bag was a cellulose ester membrane, hydrophilic, andhad a molecular weight cut-off (MWCO) of 100 Da; and the hydraulicretention time of the dialysate in the sewage sample was 4 h.Thereafter, the concentrations of the total dissolved nitrogen (TDN),ammonia nitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) and nitrite nitrogen(NO₂ ⁻) were measured using potassium persulfate oxidation-ionchromatography, salicylic acid-hypochlorite spectrophotometry, ionchromatography and N-(1-naphthyl)-ethylenediamine spectrophotometry,respectively, and recorded as C_(TDN(II)), C_(NH4) ⁺ _((II)), C_(NO3) ⁻_((II)), and C_(NO2) ⁻ _((II)), respectively.

4. The concentration of the dissolved organic nitrogen (DON) in thesewage sample was calculated as follows: DON=C_(TDN(II))—C_(NH4) ⁺_((II))—C_(NO3) ⁻ _((II))—C_(NO2)-_((II)). The DON in the sample 5 was1.05 mg/L. To ensure the accuracy and reliability of the measurement,the measurement was performed three times and the measured values wereaveraged. The results are shown in FIG. 2.

As shown in FIG. 2, when the sewage samples 1-4 were not pretreatedusing the method described in the disclosure, the standard deviation ofthe measured values was large, and thus the concentration of the DON inthe sewage cannot be concluded, or even a negative value was obtained.According to the measurement method as described in the disclosure, themeasured value of the DON concentration of the sample 5 was 1.03±0.03mg/L, and the standard error between the measured value and the truthvalue (0.99 mg/L) is 4.04%. The standard error between the DONconcentration measured using conventional methods and the truth value inthe sample 5 is 33.33%. Therefore, the method of measurement of theconcentration of DON is accurate. The standard deviation of the repeatedtests is within 10%.

It will be obvious to those skilled in the art that changes andmodifications may be made, and therefore, the aim in the appended claimsis to cover all such changes and modifications.

What is claimed is:
 1. A method of measuring a concentration ofdissolved organic nitrogen in sewage, the method comprising: (1)filtering a sewage sample using a filter membrane to obtain a filteredsewage sample; (2) measuring concentrations of total dissolved nitrogen(TDN), ammonia nitrogen (NH₄ ⁺), and nitric nitrogen (NO₃ ⁻) in thefiltered sewage sample, respectively designated as C_(TDN(I)), C_(NH4) ⁺_((I)) and C_(NO3) ⁻ _((I)); (3) according to (C_(NH4) ⁺ _((I))+C_(NO3)⁻ _((I)))/C_(TDN(I)) and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I)), performingone of the following to measure concentrations of nitrogen-containinggroups in the sewage sample; (3.1) when (C_(NH4) ⁺ _((I))+C_(NO3) ⁻_((I)))/C_(TDN(I))<0.7, measuring a concentration of nitrite nitrogen(NO₂ ⁻) in the filtered sewage sample, designated as C_(NO2) ⁻ _((I)),and obtaining a concentration of dissolved organic nitrogen (DON) in thesewage sample as follows: DON=C_(TDN(I))—C_(NH4) ⁺ _((I))—C_(NO3) ⁻_((I))—C_(NO2) ⁻ _((I)); (3.2) when (C_(NH4) ⁺ _((I))+C_(NO3) ⁻_((I)))/C_(TDN(I))≥0.7 and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))≥1,dialyzing the filtered sewage sample in a suspended dialysis bag forbetween 22 and 26 hours to obtain a dialyzed sewage sample, andmeasuring concentrations of total dissolved nitrogen (TDN), ammonianitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) and nitrite nitrogen (NO₂ ⁻)in the dialyzed sewage sample, respectively designated as C_(TDN(II)),C_(NH4) ⁺ _((II)), C_(NO3) ⁻ _((II)), and C_(NO2) ⁻ _((II)), andobtaining a concentration of dissolved organic nitrogen (DON) in thesewage sample as follows: DON=C_(TDN(II))—C_(NH4) ⁺ _((II))—C_(NO3) ⁻_((II))—C_(NO2)-_((II)); and (3.3) when (C_(NH4) ⁺ _((I))+C_(NO3) ⁻_((I))/C_(TDN(I))≥0.7 and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))<1, dialyzingthe filtered sewage sample in a suspended dialysis bag for between 34and 38 hours to obtain a dialyzed sewage sample, and measuringconcentrations of total dissolved nitrogen (TDN), ammonia nitrogen (NH₄⁺), nitric nitrogen (NO₃ ⁻) and nitrite nitrogen (NO₂ ⁻) in the dialyzedsewage sample, respectively designated as C_(TDN(III)), C_(NH4) ⁺_((III)), C_(NO3) ⁻ _((III)), and C_(NO2) ⁻ _((III)), and obtaining aconcentration of dissolved organic nitrogen (DON) in the sewage sampleas follows: DON=C_(TDN(III))—C_(NH4) ⁺ _((III))—C_(NO3) ⁻_((III))—C_(NO2) ⁻ _((III)).
 2. The method of claim 1, wherein thefilter membrane has a pore size of 0.45 μm.
 3. The method of claim 1,wherein the suspended dialysis bag in (3.2) and (3.3) is a celluloseester membrane and has a molecular weight cut-off of 100-500 Da; and ahydraulic retention time of a dialysate is 4 h.
 4. The method of claim1, wherein the concentrations of the total dissolved nitrogen (TDN),ammonia nitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) and nitrite nitrogen(NO₂ ⁻) are measured using potassium persulfate oxidation-ionchromatography, salicylic acid-hypochlorite spectrophotometry, ionchromatography and N-(1-naphthyl)-ethylenediamine spectrophotometry,respectively.
 5. The method of claim 3, wherein the suspended dialysisbag in (3.2) and (3.3) is hydrophilic.
 6. The method of claim 1, whereinthe sewage sample of (1) is a municipal sewage sample extracted from ananaerobic stage or an aerobic stage of a wastewater treatment plant. 7.A method of measuring a concentration of dissolved organic nitrogen insewage, the method comprising: (1) filtering a sewage sample using afilter membrane to obtain a filtered sewage sample; (2) measuringconcentrations of total dissolved nitrogen (TDN), ammonia nitrogen (NH₄⁺), and nitric nitrogen (NO₃ ⁻) in the filtered sewage sample,respectively designated as C_(TDN(I)), C_(NH4) ⁺ _((I)) and C_(NO3) ⁻_((I)); (3) according to (C_(NH4) ⁺ _((I))+C_(NO3) ⁻ _((I)))/C_(TDN(I))and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I)), performing one of the followingto measure concentrations of nitrogen-containing groups in the sewagesample; (3.1) when (C_(NH4) ⁺ _((I))+C_(NO3) ⁻ _((I))/C_(TDN(I))<0.7,measuring a concentration of nitrite nitrogen (NO₂ ⁻) in the filteredsewage sample, designated as C_(NO2) ⁻ _((I)), and obtaining aconcentration of dissolved organic nitrogen (DON) in the sewage sampleas follows: DON=C_(TDN(I))—C_(NH4) ⁺ _((I))—C_(NO3) ⁻ _((I))—C_(NO2) ⁻_((I)); (3.2) when (C_(NH4) ⁺ _((I))+C_(NO3) ⁻ _((I)))/C_(TDN(I))≥0.7and C_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))≥1, dialyzing the filtered sewagesample in a suspended dialysis bag for critically between 22 and 26hours to obtain a dialyzed sewage sample, and measuring concentrationsof total dissolved nitrogen (TDN), ammonia nitrogen (NH₄ ⁺), nitricnitrogen (NO₃ ⁻) and nitrite nitrogen (NO₂ ⁻) in the dialyzed sewagesample, respectively designated as C_(TDN(II)), C_(NH4) ⁺ _((II)),C_(NO3) ⁻ _((II)), and C_(NO2)—^((II)), and obtaining a concentration ofdissolved organic nitrogen (DON) in the sewage sample as follows:DON=C_(TDN(II))—C_(NH4) ⁺ _((II))—C_(NO3) ⁻ _((II))—C_(NO2)-_((II)); and(3.3) when (C_(NH4) ⁺ _((I))+C_(NO3) ⁻ _((I)))/C_(TDN(I))≥0.7 andC_(NO3) ⁻ _((I))/C_(NH4) ⁺ _((I))<1, dialyzing the filtered sewagesample in a suspended dialysis bag for critically between 34 and 38hours to obtain a dialyzed sewage sample, and measuring concentrationsof total dissolved nitrogen (TDN), ammonia nitrogen (NH₄ ⁺), nitricnitrogen (NO₃ ⁻) and nitrite nitrogen (NO₂ ⁻) in the dialyzed sewagesample, respectively designated as C_(TDN(III)), C_(NH4) ⁺ _((III)),C_(NO3) ⁻ _((III)), and C_(NO2) ⁻ _((III)), and obtaining aconcentration of dissolved organic nitrogen (DON) in the sewage sampleas follows: DON=C_(TDN(III))—C_(NH4) ⁺ _((III))—C_(NO3) ⁻_((III))—C_(NO2) ⁻ _((III)).
 8. The method of claim 7, wherein thefilter membrane has a pore size of 0.45 μm.
 9. The method of claim 7,wherein the suspended dialysis bag in (3.2) and (3.3) is a celluloseester membrane and has a molecular weight cut-off of 100-500 Da; and ahydraulic retention time of a dialysate is critically 4 h.
 10. Themethod of claim 7, wherein the concentrations of the total dissolvednitrogen (TDN), ammonia nitrogen (NH₄ ⁺), nitric nitrogen (NO₃ ⁻) andnitrite nitrogen (NO₂ ⁻) are measured using potassium persulfateoxidation-ion chromatography, salicylic acid-hypochloritespectrophotometry, ion chromatography and N-(1-naphthyl)-ethylenediaminespectrophotometry, respectively.
 11. The method of claim 9, wherein thesuspended dialysis bag in (3.2) and (3.3) is hydrophilic.
 12. The methodof claim 7, wherein the sewage sample of (1) is a municipal sewagesample extracted from an anaerobic stage or an aerobic stage of awastewater treatment plant.